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Looks like Cabelas might have some http://www.cabelas.ca/index.cfm?pageID=71&§ion=1195§ion2=1377&ID=4896
However, looks like it'd run you close to $100 for a single ring.
That seems like robbery to me, so I'm willing to help a brother out. I've got a #4 (grey)that came with my GSR that I'm not using. If I understand this correctly it should also work for your m77. Send me a PM if you're interested.
Brownells has a more reasonably priced selection http://www.brownells.com/.aspx/pid=24396/Product/FACTORY-SCOPE-RINGS
We sell Ruger individual rings (thats how they come now from Ruger). There are several finishes (matte, gloss grey)in both silver and black, please specify what you are looking for, most are in stock. Black $34 and silver $45. Phil.